The present invention relates to a stretched mask for a color picture tube, which can be used for any type of color picture tube, e.g. a shadow mask tube or an aperture grille tube, in color television and computer color displays.
In color picture tubes for color television and color displays, a mask for color selection is used so that electron beams are applied to predetermined phosphors. As the color selection mask, a shadow mask formed from a metal sheet provided with a large number of small holes or an aperture grille provided with a large number of slits is used. When a color picture tube is used continuously for a long period of time, the shadow mask or the aperture grille is heated because accelerated electrons collide against it, and distorted by thermal expansion. This may cause the electron beams to be gradually displaced relative to the phosphor screen, resulting in color shift in the colored image.
As a color selection mask for a color picture tube, a stretched color discrimination mask like an aperture grille, which is stretched on a firm frame, is used as well as a member pressed like a shadow mask.
The stretched color discrimination mask is formed as follows. A hot-rolled low-carbon steel strip containing carbon in units of 0.0001% is cold-rolled to a sheet having a thickness of 0.02 mm to 0.30 mm. After a large number of grid elements have been formed in the steel sheet by etching, the steel sheet is welded to a frame placed under pressure applied in a direction reverse to the stretching direction. Then, the pressure is removed to form tension by the restoring force of the frame. Thereafter, to prevent the generation of secondary electrons, heat radiation, formation of rust, etc., the mask stock is subjected to heat treatment for 10 to 20 minutes in an oxidizing atmosphere at 450xc2x0 C. to 470xc2x0 C., thereby blackening the surface of the mask.
In a color picture tube, the temperature of the color discrimination mask is raised by irradiation with electron beams. However, the color discrimination mask reaches approximately thermal equilibrium about 2 hours after starting energization. In the case of a color discrimination mask made of a low-carbon steel sheet, thermal expansion occurs to a considerable extent as the temperature rises. Therefore, the color discrimination mask needs to be stretched under high tension so that the stretched color discrimination member will not slacken during use. For stretching under high tension, a member of high strength is needed, and a frame for mounting the stretched mask also needs high strength.
Under these circumstances, a method has been conceived in which deformation or the like due to the rise in temperature is prevented by using a metallic material having a low coefficient of thermal expansion. However, an Invar material consisting of a nickel-iron alloy, which is a typical metallic material of low thermal expansion used for shadow masks of color picture tubes, has no sufficiently high strength and is therefore incapable of providing a sheet that endures a tension required for stretching. Accordingly, it is impossible to obtain a satisfactory sheet stock for producing a stretched color discrimination mask.
Meanwhile, the stretched color discrimination member needs to reduce the thickness of the metallic material in comparison to a color discrimination member press-formed in conformity to the curved surface of a color picture tube. Therefore, the color discrimination member made of a low-carbon steel sheet cannot provide a satisfactory magnetic shielding effect.
An object of the present invention is to provide a stretched color selection device for a color picture tube that has a high magnetic shielding effect and a low coefficient of thermal expansion and hence allows the stretching tension to be reduced.